In a directly under lighting device, a light diffusion plate is used to make light from a light source (halogen lamp, LED, laser etc.) gentle for eyes. Note that, however, in order to prevent deterioration in the quality of lighting, it is necessary to reduce lighting irregularities and coloring due to direct transmission of lamp image and occurrence of interference pattern. Therefore, the light diffusion plate is required to have a a characteristic of uniformly diffusing light as well as a characteristic of high transmittance. An opaque white colored light diffusion plate is the most typically known light diffusion plate. The light diffusion properties of the opaque white colored light diffusion plate was obtained by adding light diffusion particles such as glass, acrylic, barium sulfate, titanium dioxide, ammonium oxide and silicon rubber to base resin with a high light transmittance such as acrylic resin and polycarbonate resin, and irregularly reflecting incident light by those light diffusion particles.
It is necessary to add a large quantity of light diffusion agent to the opaque white colored light diffusion plate in order to impart light diffusibility to the light diffusion plate that is enough to make lamp image invisible. In this case, a light transmittance of the light diffusion plate is to be lowered. Therefore, in many of light diffusion plates suitable for directly under lighting systems, which are increasing in number rapidly, the type, the diameter and the blending volume of light diffusion particles are regulated to achieve a high transmittance and high light diffusion (Patent Literature 1).
Further, as light diffusion plates with no use of light diffusion particles, a light diffusion plate having a microscopic random concave and convex pattern formed by laser interference exposure (Patent Literature 2) and a light diffusion plate having a microscopic random concave and convex pattern formed by sandblasting, embossing or the like (Patent Literature 3) are known. In addition, a light diffusion plate having a random concave and convex pattern which is obtained by placing a variety of microlenses according to a probability distribution by laser machining or the like (Patent Literature 4) is known.